1. Field of Invention
The present invention relates to a fixing apparatus for fixing images formed on paper and an image forming apparatus provided with this fixing apparatus.
2. Description of Related Art
In general, an image forming apparatus as an electrophotographic system (printer, copying machine, facsimile or the like) is provided with a fixing apparatus for applying heat and pressure to a sheet to fix a toner image transferred to the sheet. This fixing apparatus includes a heating unit for heating and melting toner on a sheet and a pressing unit for pressing the sheet against the heating unit.
The pressing unit of the fixing apparatus consists, for example, of a fixing roller and a pressure roller which is urged against the fixing roller with a predetermined load. A nip portion is formed between the fixing roller and the pressure roller which is directly or indirectly urged against the fixing roller to hold and convey a sheet therebetween.
The heating unit of the fixing apparatus consists of a heat source (for example, halogen heater) contained in the pressure roller, and an endless fixing belt which is wound around the fixing roller (heating belt type). In this case, a nip portion is formed by urging the pressure roller against the fixing roller through the fixing belt. Alternatively, the fixing roller may incorporate a heat source, and serves itself as a heating unit (heating roller type). In this case, the pressure roller is urged directly against the fixing roller while a nip portion is formed therebetween.
The image forming apparatus having such a fixing apparatus develops toner images on photoreceptor drums in correspondence with image data, and transfers the toner images on a sheet. The sheet with the transferred toner images is conveyed to the fixing apparatus, and passed through the nip portion to fix the toner images with heat and pressure.
This kind of fixing apparatus is described, for example, in Japanese Patent Published Application No. 06-250560, Japanese Patent Published Application No. 10-221999, and Japanese Patent Published Application No. 09-138598.
In the case of the fixing apparatuses described in Japanese Patent Published Application No. 06-250560 and Japanese Patent Published Application No. 10-221999, a nip portion is formed with a fixing roller on which is partly wound an endless belt running around a plurality of rollers. The fixing apparatus includes a pressure roller located in contact with the fixing roller through the endless belt from the inside of the endless belt at the exit of the nip portion. The fixing apparatus prevents displacement of images by exerting a braking force on the endless belt conveyed on the pressure roller in order to remove the difference in the conveyance speed between the pressure roller and the fixing roller. On the other hand, in the case of the fixing apparatus described in Japanese Patent Published Application No. 09-138598, a heat-resistant belt is supported by a plurality of rollers around which this belt is wound. This fixing apparatus includes a pressure roller urged in contact with a plurality of rollers through the heat-resistant belt. A tension is given to the heat-resistant belt by controlling the plurality of rollers.
When fixing toner in the fixing apparatus, the surface of a sheet bearing an unfixed toner image comes in direct contact with a heating unit (fixing belt or fixing roller). Accordingly, a latent image may be formed on the heating unit with wax, which is soaked from toner and attached to the heating unit (fixing belt or fixing roller), and may appear on the next image. More specifically, when fixing toner to form the next image, the wax attached to the heating unit appears as the unevenness of gloss (referred to as a gloss memory) corresponding to the unevenness of the attached wax amount.
There is a demand to clear such a gloss memory when fixing toner in a fixing apparatus and improve the image quality. However, such a gloss memory cannot be prevented from occurring in the fixing apparatuses described in Japanese Patent Published Application No. 06-250560, Japanese Patent Published Application No. 10-221999, and Japanese Patent Published Application No. 09-138598.
The applicant has proposed therefore a fixing apparatus comprising: a fixing side member configured to rotate; a back side member configured to rotate in contact with the outer peripheral surface of the fixing side member under pressure, and cooperate with the fixing side member for forming a fixing nip portion which holds and conveys a sheet with a toner image therebetween; a drive motor configured to rotate the back side member; and an effective braking force generation unit configured to generate an effective braking force in the direction to hinder rotation of the fixing side member to set a differential speed between the surface speed of the fixing side member and the surface speed of the back side member.
With this fixing apparatus, it is possible to make the fixing side member and the sheet slip on each other at the fixing nip portion by the differential speed which is set between the surface speed of the fixing side member and the surface speed of the back side member, and therefore solve the gloss memory problem.
Incidentally, the effective braking force generation unit of this fixing apparatus includes a motor which generates a braking force exerted on the fixing side member to hinder rotation thereof. However, a sheet is passed through the fixing nip portion of this fixing apparatus on which a braking force is being exerted by the motor. Because of this, the motor functions as an electric generator. The temperature of the motor therefore tends to rise and be broken.